A relaxation oscillator is one in which one or more voltages or currents change suddenly at least once during each cycle. The oscillator circuit is arranged so that during each cycle, energy is stored in and then discharged from a reactive element such as a capacitor or an inductor. The charging and discharging processes occupy different time intervals. Such a relaxation oscillator has an asymmetrical output waveform rather than a sinusoidal waveform, e.g., a sawtooth type waveform. Common types of relaxation oscillators include multi-vibrator and uni-junction transistor oscillators.
One of the oldest relaxation oscillators uses a neon tube as V.sub.s of neon tube 16, the neon tube 16 abruptly begins to conduct, and in shown in FIG. 1. Current from a supply 10 flows through resistor 14 to charge capacitor 18. When capacitor 18 reaches a striking voltage the process, discharges the capacitor 18. When the capacitor discharges through the holding voltage V.sub.h of the neon tube 16, the neon tube 16 ceases to conduct. As a result, capacitor 18 resumes charging. The overall result is the sawtooth type waveform which oscillates between V.sub.s and V.sub.h.
A bipolar junction transistors-based relaxation oscillator may be constructed using bipolar junction transistors rather than a neon tube. An PNP bipolar junction transistor 20 may be connected "back-to-back" with an NPN bipolar junction transistor 22. More specifically, the base of transistor 20 is connected to the collector of transistor 22, and the base of transistor 22 is connected to the collector of transistor 20. Preferably, the collector-to-base junction of one of these transistors 20 and 22 has a well-defined reversed breakdown voltage such as is exhibited by a Zener diode. Below the Zener breakdown voltage, the collector-base junction is reverse-biased and therefore does not conduct. Above the Zener breakdown voltage, the junction is forward-biased and supports relatively high currents. The Zener diode alone, however, cannot produce a relaxation oscillation because it does not exhibit hysteresis.
Nonetheless, the circuit of FIG. 2 does exhibit some hysteresis. The voltage across the collector-base junction of the PNP transistor may be represented by an intrinsic Zener diode 24. When that collector-base junction voltage is less than a forward bias voltage, transistor 20 is basically non-conducting exhibiting only a small leakage current from the collector of PNP transistor 20 to the base of NPN transistor 22. Similarly, NPN transistor 22 passes only a small collector leakage current to the base of PNP transistor 20. At low currents, the gain of transistors 20 and 22 is less than unity so that the leakage currents are not magnified. Essentially, the transistors can be viewed as a very high impedance, e.g., essentially nonconducting. As the voltage V across the transistors increases as capacitor 18 charges from voltage supply 12 through resistor 14, the leakage currents increase quite sharply when the Zener or breakdown voltage is reached. At such higher currents, transistor gain is larger than unity. Consequently, the collector current of transistor 22 flowing into the base of transistor 20 is amplified thereby increasing the drive of transistor 20 resulting in a collector current larger than that of transistor 22. This larger collector current flows into the base of transistor 22 and is also amplified ultimately driving transistor 22 harder. Those increasing currents essentially "snowball" so that a relatively high current flows through the transistors to rapidly discharge capacitor 12. This current is initiated by the onset of Zener breakdown and is maintained during discharge by the current amplification which together produce large hysteresis.
When capacitor 12 is sufficiently discharged, (i.e., the value of V has decreased), to a point where the current flow through transistors 20 and 22 returns to the level where their respective gains are less than unity, the snowball effect ceases. The transistors return to the almost nonconducting, low leakage current state allowing capacitor 18 to recharge. The cycle of charge, discharge, and recharge generates an oscillating voltage output V.
FIG. 3 shows how a PNP bipolar junction transistor and an NPN bipolar junction transistor can be merged when the collector of each transistor is connected to the base of the other transistor to form a four-layer PNPN diode. Such four layer structures are often accidentally formed when making CMOS integrated circuits and are considered undesirable because they result in the so-called "substrate latch-up" problem. As a result, steps are usually taken in CMOS production processes to avoid the formation of accidental four layer diodes. Another significant problem associated with such four layer devices is the difficulty of controlling their characteristics. In other words, the breakdown voltage at which the four-layer diode conducts depends very much on independent hard-to-control variables such as temperature and doping content of the semiconductor material which are hard to produce consistently. Therefore, it is difficult to predict and control the output characteristics of an oscillator formed using this type of PNPN diode structure.
It is therefore an object of the present invention to provide an equivalent of a four-layer PNPN diode device which can be readily controlled in a CMOS type configuration. In particular, it is an object of the present invention to construct such a device to function as a simple, current-controlled oscillator.
The present invention provides an electric oscillator of reduced complexity which is also suitable for construction on a silicon integrated circuit. A P-channel field effect transistor (FET) operated in an enhancement mode includes source, drain, and gate electrodes. An N-channel field effect transistor (FET) operated in an enhancement mode is connected to the P-channel field effect transistor in complementary fashion in a complementary metal oxide silicon (CMOS) circuit. More specifically, the drain electrode of the P-channel FET is connected to the gate electrode of the N-channel FET, and the drain electrode of the N-channel FET is connected to the gate electrode of the P-channel FET. A first capacitor is connected across the P-channel and N-channel FETs such that the capacitor is repetitively charged and discharged through the CMOS circuit in response to a current generated by a current source.
The current generated by the current source charges the capacitor up to a striking voltage at which the P-channel and N-channel FETs conduct current to discharge the capacitor down to a holding voltage at which the P-channel and N-channel FET cease conducting current. In essence, the current generated by the current source controls the frequency of the output signal generated by the oscillator.
A resistor is connected between the drain electrodes of the P-channel and N-channel FETs. A second capacitor is connected between the drain electrodes of the P-channel and N-channel FETs in shunt across the resistor. The resistor provides a voltage drop across the gate and drain electrodes of both FETs such that threshold voltage is sufficient to cause the complementary FETs to conduct when the capacitor is charged to the striking voltage. Conversely, the resistor ensures that the voltages applied to the gate of both transistors are less than their respective threshold turn-on voltages when the oscillator output voltage reaches its holding value. The shunt capacitor maintains a smooth transition between conduction and non-conduction for the FETs.
The current-controlled relaxation oscillator in accordance with the present invention includes a complementary metal oxide silicon (CMOS) circuit equivalent in function to a four-layer junction diode having three P-N junctions. The CMOS circuit is connected to positive and negative terminals of a voltage supply and to the storage capacitor. The current source is connected to the negative terminal of the voltage supply and to ground. The current generated by the current source charges and discharges the capacitor to control frequency that depends on the current source current such that the voltage output of the oscillator oscillates between a striking voltage at the control frequency. As a result, the present invention provides an easily manufactured and controlled four-layer junction diode suitable for low cost, real world applications.
One application of the present invention is to radio communications. The radio includes a microphone for detecting an acoustic signal, a reduced complexity current controlled oscillator connected to the microphone for generating an oscillator output signal representing the acoustic signal, and a radio transmitter for transmitting over an antenna of radio output signal based on the oscillator output signal. A digital frequency modulation discriminator may be connected to the oscillator for generating a sample of digital representation of an instantaneous frequency of the oscillator output signal, thus providing a simple technique for digitizing an audio signal.